<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <script>
        function isRunYear(year) {
            if(year % 4 === 0 && year % 100 !== 0 || year % 400 === 0) {
                return true
            } else {
                return false
            }
        }
        // var result  = isRunYear(1800)
        // console.log(result);
        function day(year) {
            var result = isRunYear(year)
            if(result) {
                alert('2月是29天')
            } else {
                alert('2月是28天')
            }
        }
        day(2000)
    </script>
</body>
</html>